In article <48pm5q$m9o@due.unit.no>, aslef@alkymi.unit.no says...
>Yes, I know that all zoom lenses have variabel focal length. But in >this case, the focal lenght is variabel when focusing.
(examples deleted)
>My questions are:
>Are there anyone who have other experiences about this?
>Are there anyone who knows something at all about this?
>Are there just zoom lenses or are there other lenses (ex. tele
>lenses with internal focusing) with this type of variabel focal length?

In classic zoom design, focal-length is changed by varying the distance
between the front and rear sections of the lens. Focus is accomplished by moving the front section relative to the rear while keeping the rear section stationary. Effectively, the lens shortens in focal-length
as you focus toward nearer objects, while the barrel remains the same
(shorter lens in same barrel-length = closer focus). (Some new zoom designs use other sections for focus, and may not show the same effect.)
A non-zoom non-IF lens focusses by keeping the focal-length constant
while changing the barrel-length (and will have a narrower angle of
view on film at close focus, though the change in angle is not as
pronounced as it is in most zooms [one reason zooms are harder to focus than single focal-length lenses: magnification changes with focus]).
IF lenses focus by holding the barrel-length constant while changing the focal-length. Some macro lenses combine IF and non-IF to keep both the barrel-length change and focal-length change reasonable.
Hope this helps.